There is an updated pull request by tornaria against master on the void-packages repository

https://github.com/tornaria/void-packages isl
https://github.com/void-linux/void-packages/pull/33332

isl, isl15: update homepage and distfile url
It seems gforge.inria.fr is down.

Per README, the project repo is on repo.or.cz, but the original tarballs
are not available. The available tarballs need autotools to build.

Also removed 'bootstrap=yes'.

A patch file from https://github.com/void-linux/void-packages/pull/33332.patch is attached